Any reason you decided on 15x15 ?

and yes, cutting ally needs a very rigid build, and has to be designed in from the start.

Spindles ... it depends on your budget ? but a cheapish router will get you started. I've just bought a £25 one from Aldi.
ALDI - Sunday Special Buys 9th September 2012
I'll be fitting it to my build soon, as my Bosch router is about 20 yrs old and needs the bearings replacing.

You can change the router mount to a spindle mount at any time if you make it as a 'bolt on' to the main carriage.
Design the carriage so it has no interference from the router/spindle at the end of the gantry, else you'll loose some working area if you put anything bigger on later.